There is a difference between what is illegal and what is unethical.

Yes. He has no technical obligation to return the money if he doesn't want to. But, it's not unreasonable for Republicans to want their money back and I believe it would be ethical for him to return it.

Not doing things exactly as you want is one thing. Republicans donating to an individual supporting party efforts, only to have the guy use that money for the interests of another party is different than your example.